This invention relates to packings or seals for hydraulic cylinders, the seals being formed of an ethylene-.alpha.-olefin-diene copolymer rubber composition. In particular the invention is suitable for application to automotive hydraulic cylinders such as brake and clutch master cylinders, wheel cylinders of drum brake systems and disc cylinders of disc brake systems.
As seal materials for automotive hydraulic cylinders natural rubber and styrene-butadiene rubber (SBR) have long been used, but in recent years ethylene-propylene-diene terpolymer rubbers (EPDM), which are superior in high- and low-temperature resistance and aging resistance, have been prevailing as the engine room temperatures have been rising and as brake fluids of higher boiling points have been employed. In some cases propylene in EPDM is replaced by another .alpha.-olefin.
Generally EPDM is not good in kneadability, particularly with rolls, and this offers a problem to industrial preparation of EPDM rubber compositions. A compensatory measure is using EPDM relatively low in Mooney viscosity, but this is unfavorable for other properties of EPDM since reducing Mooney viscosity means reducing molecular weight. Using low molecular weight EPDM it is difficult to obtain rubber seals which can long remain stable and effective in automotive hydraulic cylinders.
As seal materials for automotive hydraulic cylineres EPDM rubber compositions are desired to be good in kneadability and, by molding and curing, can provide seals low in susceptibility to yielding (loss of resiliency) by fatigue. However, such desire cannot easily be met. As is known, for improving resistance to yielding by fatigue there is the need of using an EPDM of high molecular weight with narrow distribution of molecular weight. However, this is contradictory to the desire for improved kneadability and increases the material cost. Besides, if this measure is taken indeliberately the resultant EPDM compositions will possibly suffer from bad dispersion of a particulate filler such as carbon black.